Organic Cosolutes as Stabilizers of Phosphoenolpyruvate Carboxylase in Storage: an Interpretation of Their Action

Abstract

Phosphoenolpyruvate carboxylase (EC 4.1.1.3 1) is rapidly inactivated in dilute extracts from leaves of the C₄ grass Cynodon dactylon (L.) Pers. Physiological osmotica (glycerol, betaine, proline, sorbitol and sucrose) and synthetic inert polymers like polyvinylpyrrolidone and polyethylene glycol prevent inactivation in a concentration-dependent manner. In a partially purified preparation (10.5 µmol CO₂ min^-1 mg^-1), lability to inactivation and the minimum levels of cosolvents needed for complete protection become higher as the protein is diluted. Exogenous protein (bovine serum albumin) was much less effective than native protein in stabilising activity. The results could be interpreted on the basis of the exclusion volume theory and the assumption that the catalytic activity is mainly due to a polymeric form which is maintained only at high concentrations of the enzymic protein.